In typical fields of application of radioscopy such as, for example, X-ray inspection or medical X-ray diagnostics, the resolution which can be achieved in the radioscopy plays an important role. Good resolution is achieved when detector arrays with detector elements of small area which are as close to one another as possible and a device for closely limiting the wide solid angle at which the X-ray radiation can impinge on the respective detector element, which is arranged in front of these detector elements, are used. In the ideal case, this device, known as collimator, only allows the X-ray radiation propagating on a straight line connection between the focus of the X-ray tube used and the respective detector element to pass and absorbs X-ray radiation which is incident an another angle due to scattering.
Due to its history, the stray radiation does not contribute to the image information and leads to a distinct impairment of the signal/noise ratio and of the achievable resolution of the X-ray image if it impinges on the detector elements with full force. By using suitable collimators which, as a rule, are adapted to the geometric relationships of the respective X-ray system, particularly the arrangement of the X-ray tube and X-ray detectors,
the proportion of stray radiation reaching the detector elements can be clearly reduced so that, in many cases, usable X-ray images are only obtained by this.
Collimators include numerous absorption elements for X-ray radiation, which are separated from one another by a filler and support material and are aligned approximately in parallel or oriented towards a common focus, the focus of the X-ray tube. In present-day X-ray CT installations, collimators are still used, as a rule, which have lead strips extending approximately in parallel with one another or aligned towards the X-ray focus, between which paper strips are inserted as filler and support material. In many cases, the distance of the lead strips is adjusted during the production of the collimators in such a manner that the lead strips are located as accurately as possible above the partitions of the fluorescent arrays at the detector end when the collimator is used.
The collimators must, therefore, be produced with great mechanical precision. The alignment onto the focus of the X-ray tube implemented in part also requires an elaborate production process. Due to these high requirements for precision, the production of the collimators causes high costs. A two-dimensional collimation of the X-ray radiation as is required when two-dimensional detector arrays are used cannot be achieved with such collimators, either.
From DE 197 26 846 C1, a collimator is known in which the distance of the absorption elements which are also strip-shaped here and which are aligned in parallel with one another continuously increases from the center of the collimator towards the edge. At the same time, the width of the absorption elements is increased towards the edge. Constructing the collimator in this manner makes it possible to implement an absorption characteristic which is largely uniform over the entire collimator width.
However, the requirements for manufacturing precision are also high in this case.
From DE 199 20 301 C2, a further collimator is known in which the absorption elements extend essentially radially with respect to a center in spaced-apart rows. The variation and the arrangement of the absorption elements are predetermined in accordance with a particular rule in this collimator. The support material used is silicon into which holes are etched in accordance with the required variation of the rows of absorption elements. Into these holes, pin-shaped absorption elements of lead are inserted. This collimator, too, requires that very high precision is maintained during the production which is achieved, in particular, by the proposed manufacturing technique with silicon as support material.
U.S. Pat. No. 5,263,075 A describes a collimator which allows two-dimensional collimation of the incident X-ray radiation. The collimator is produced from a glass fiber bundle from which individual disc-shaped sections are sawn out. The cores of the individual glass fibers are etched out so that capillary passage channels are produced for the X-ray radiation. The glass material is subsequently doped with up to 60% lead in the form of lead oxide so that an increased X-ray absorption is achieved outside the passage channels. Due to the etching and doping steps required in this arrangement, the production of this collimator is also relatively expensive.